The mechanism of color generation in poly(ethylene terephthalate)/polyamide blends

Color generation in blends of poly(ethylene terephthalate) and poly(m-xylylene adipamide), PET/MXD6, have been studied. Techniques, including TGA-MS, 1H, 13C NMR and MALDI-MS, were used to characterize reaction products which result in the observed color generation; both the polymer system and model compounds were used to understand the underlying chemistry. The proposed mechanism, which is different from that generally proposed for yellowing of polyamides alone, is dependent upon acetaldehyde (which itself is a by-product of PET processing). Acetaldehyde is proposed to condense with polyamide amino end groups to produce intermediate (and uncolored) imines. These intermediate imines in turn condense with additional equivalents of acetaldehyde to produce conjugated imines, which are the colored chromophores.